1. Field of Invention
The embodiment described below relates to a multi-carrier communication apparatus, an integrated circuit, a multi-carrier communication system, and a multi-carrier communication method, in which communication is performed by using a plurality of carriers.
2. Description of Related Art
A transmission method using a plurality of sub-carriers, such as an OFDM (Orthogonal Frequency Division Multiplexing) method, has a major advantage that high quality communication is possible even when the communication is performed through a rough transmission line, and has been used for not only wireless communication but also wired communication such as power line communication.
A multi-carrier communication apparatus that performs such communication by using a plurality of sub-carriers transmits transmission bit data on a transmitting side by converting the bit data to symbol data; performing a symbol mapping according to the symbol data; converting the data to time-domain data via an inverse FFT transform or an inverse wavelet transform; performing a parallel-serial transform; and converting the data to a base-band analog signal via a DA conversion. The multi-carrier communication apparatus receives reception bit data on a receiving side by converting a received signal to a digital signal via an AD conversion; performing a serial-parallel transform; converting the data to frequency-domain data via an FFT transform or a wavelet transform; and performing a demapping.
Such multi-carrier communication apparatus has a carrier detection function that determines whether another apparatus is transmitting a signal to a transmission line, and performs a transmission process when another apparatus is not using the transmission line and a transmission request is received.
Carrier detection of the multi-carrier communication apparatus, as shown in Japanese Laid-Open Patent Publication 2001-94527, for example, is performed based on an AD-converted reception signal. As shown in US 2005-037722 A1, carrier detection may also be performed based on a signal obtained by converting AD-converted digital data to frequency-domain data.
In the case where carrier detection is performed based on an AD-converted reception signal, correlation of a signal in a time domain is used, and carrier detection can be generally realized by using a simple circuit or by performing a simple data processing. Carrier detection based on a signal that has been converted to frequency-domain data uses correlation between sub-carriers in a frequency domain, and therefore has high detection accuracy.
However, these methods of carrier detection are all based on correlations of signals existing on the transmission line, so that a carrier may not be detected even when a signal exists on the transmission line. Therefore, communication may not be performed for a long period of time when signal collision occurs due to nearly simultaneous transmission of data from different terminals to the same transmission line.
FIGS. 10(a)-10(b) and FIGS. 11(a)-11(b) are time charts explaining signal collisions. In the case where transmission signals are simultaneously output from terminal A and terminal B at time t1 shown in FIG. 10(a), the signals are not received normally by other terminals. Therefore, terminal A, which terminated transmission at time t2, intends to retransmit, because there is no response to the transmitted signal. In such case, a carrier often cannot be detected, so that it is determined that there is no signal on the transmission line, and retransmission starts at time t3. Therefore, recollision occurs. This is because the transmission signal (the portion marked by the broken-line ellipse in FIGS. 10(a) and 10(b)) from terminal B, which exists on the transmission line during a period of time near time t3, corresponds to a payload portion of the transmission data, in which there is usually no correlation. Depending on the lengths of the transmission signals of terminal A and terminal B, the signal recollision can also occur at time of retransmission from terminal B, and further can also occur consecutively.
In the case where a carrier is detected when terminal A intends to retransmit at time t3, as FIG. 10(b) shows, retransmission of the signal is not performed at time t3; the retransmission is performed at time t4 after terminal B terminated signal transmission, thereby, causing no signal recollision.
As shown in FIGS. 11(a) and 11(b), the rate of occurrence of the above-described recollision can be reduced when transmission is performed by inserting a known signal (a signal with correlation, indicated by “known” in FIGS. 11(a) and 11(b)) into the payload portion of the transmission data. In the case where transmission signals are simultaneously output from terminal A and terminal B at time t1 as shown in FIG. 11(a), the signals are not received normally by other terminals. Therefore, terminal A, which terminated transmission at time t2, intends to retransmit, because there is no response to the transmission signal. In the example shown in FIG. 11(a), the transmission signal from terminal B is a signal with correlation during the period of time between time t3 and time t4, which coincides with when terminal A intends to retransmit and performs carrier detection, retransmission is therefore not performed. Retransmission is performed at time t5 without performing carrier detection.
However, in the case where terminal A and terminal B transmit with a timing as shown in FIG. 11(b), transmission data of terminal B do not contain a signal with correlation during the period of time between time t2 and time t3. Terminal A therefore retransmits at time t3 without performing carrier detection, and recollision occurs. In other words, without inserting signals with correlation into transmission data at narrow intervals, it is not possible to surely reduce the rate of recollision. Therefore, reducing the rate of recollision causes degradation of transmission efficiency.